Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are optionally combined with powder metallurgical additives, such as for example lubricants or alloying materials, and molded in a die under high pressures to produce a desired shape. After the molding step, the compacted or “green” component may undergo a sintering step to further densify the component.
Traditionally, metallurgical powder compositions include an internal or external lubricant to more easily eject a compact from a die cavity. The internal friction forces that must be overcome to remove a compacted part from the die are measured as “stripping” and “sliding” pressures.
Most conventionally known internal lubricants reduce the green strength of the compacted part. It is believed that during compaction the internal lubricant is exuded between iron and/or alloying metal particles such that it fills the pore volume between the particles and interferes with particle-to-particle bonding. As a result some shapes cannot be pressed using known internal lubricants. Tall, thin-walled bushings, for example, require large amounts of internal lubricant to overcome die wall friction and reduce the required ejection force. Such levels of internal lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. Also, internal lubricants such as zinc stearate often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive amounts of internal lubricants can lead to compacts having poor dimensional integrity, such as for example, when volatized lubricant forms soot on the heating elements of a sintering furnace.
To avoid these problems, it is known to use an external spray lubricant rather than or in addition to an internal lubricant. However, the use of external lubricants often increases the compaction cycle time and leads to less uniform compaction.
Other powder metallurgy additives include alloying materials, usually in powder form, to provide enhanced physical properties. Commonly utilized alloying powders include nickel or copper powders. At the levels used, the cost associated with these alloying additions can add up to a significant portion of the overall cost of the powder composition.
Although alloying materials may enhance select physical properties, other properties, such as for example ductility, the ability of the part to retain its shape after a strain is applied and removed, are diminished by addition of alloying materials. Copper and nickel-containing powder additions, for example, impart low ductility to finished part and thus pose certain design constraints. Additionally, certain parts manufacturers desire to limit the amount of copper and/or nickel used in the powder metallurgy compositions that are used to form compacted parts due to the environmental and/or recycling regulations that regulate the use or disposal of those parts.
Accordingly, there exists a current and long felt need in the powder metallurgical industry to develop powder metallurgical additives, such as for example, lubricants that reduce the internal friction forces that must be overcome to remove a compacted part from the die without adversely affect the physical properties of the compact. Other additives, including alloying materials are desired as alternatives to common alloying powders utilized in powder metallurgy compositions.